Given a collection of MTS, the function returns the pairwise distance matrix,
where the distance between two MTS \(\boldsymbol X_T\) and \(\boldsymbol Y_T\) is defined
as
$$d_{VAR}(\boldsymbol X_T, \boldsymbol Y_T)=||\widehat{\boldsymbol \theta}^{\boldsymbol X_T}_{VAR}-
\widehat{\boldsymbol \theta}^{\boldsymbol Y_T}_{VAR}||,$$
where \(\widehat{\boldsymbol \theta}^{\boldsymbol X_T}_{VAR}\) and \(\widehat{\boldsymbol \theta}^{\boldsymbol Y_T}_{VAR}\) are vectors
containing the estimated VAR parameters for \(\boldsymbol X_T\) and \(\boldsymbol Y_T\), respectively. If VAR models of
different orders are fitted to \(\boldsymbol X_T\) and \(\boldsymbol Y_T\), then the shortest
vector is padded with zeros until it reaches the length of the longest vector.